The invention relates to a radar system in which are provided a digital expander at the transmitter end and a pulse compression technique at the receiver end for evaluation wherein, furthermore, a digital storage clocked by a central control unit is provided for generating the transmitter pulse, from which contents via a first digital to analog converter, an analog baseband signal, and via a second digital to analog converter an analog baseband quadrature signal is derived and supplied to a quadrature modulator which provides at the output side at an intermediate frequency range a transmitter pulse to the actual transmitter via a gate controlled by the central control unit.
Radar systems which are to have a wide range as well as high distance resolution can be realized through pulse compression technology. The transmitter signal is an expanded frequency- or phase-modulated pulse which at the receiver end is compressed in a correlator which is known as compressor. In analog technique the necessary expansion of the transmitter signal has been realized in the course of time in different ways. The properties of analog delay lines, such as for example a SAW device, corresponding to the requirements for greater transmitter pulse length and sufficient side lobe suppression, are at best less than perfect and, moreover, can frequently only be realized with a high degree of complexity. For generating an expanded modulated transmitter signal, digital expander systems would be the ideal realization for the requirements in view of the pulse compression technique. Advantageously fully coherent waveforms could be achieved with digital expander systems, as could an excellent signal to noise ratio, as well as a guaranteed exchangability of the modulation code. In the most frequently used technique for the synthesis of an arbitrary waveform, the waveform is read out of the storage at constant clock frequency. A digital to analog conversion followed by an upward mixing supplies an expanded high-frequency signal. In mixing the modulating signal onto an added carrier signal (upward mixing) either double sideband or single sideband mixing can be used. Single sideband mixing does require quadrature modulation, but in many respects would be more advantageous: compared to double sideband mixing a greater signal bandwidth could be generated and the filter requirements would be considerably less critical. Of disadvantage is the limited carrier and image frequency suppression of commercially available quadrature mixers which cannot be eliminated through filters and does not always suffice for good correlation properties (side lobe suppression) in the receiver. Likewise the requisite high level stability of the carrier signal as well as the high balancing complexity is a serious disadvantage in practice. The known digital expander system therefore meets the demands such as are encountered in pulse compression technology only conditionally.